As professional and dedicated an actor as I have always heard that Sarah was, it wouldn't be hard for me to imagine that when she did actually have some time when she wasn't either preparing for or actually playing her role, she would want time alone or with someone other than the people who she had to work with every day. On the other hand, also, given her work ethic, I find it very hard to imagine her being on good terms with her costars, simply because that would make everybody's work easier.
Sarah and Alyson might not have been best friends, but I think that they were, at least, friendly in their dealings with each other. As for Sarah announcing that Season 7 would be her last year on the show, I seem to remember someone, maybe Joss, but I'm not certain, that everyone pretty much knew before Season 7 started that it would be their last year, but that they were all sworn to secrecy before the official announcement. If that's true, I can't see how Alison could have any really bad feelings about it.